Battery Solar System Design UK: Sizing, Connection and Handoff

A UK solar-plus-battery design starts with the household’s actual consumption and the service being purchased: shifting solar into the evening, charging on a time-of-use tariff, backing up selected circuits, or a combination. These uses compete for the same capacity. A battery chosen only from annual bills or bedroom count can leave important power, connection and financial questions unanswered.

This guide retains the original load-data, equipment-selection, network-submission and customer-handoff workflow. It replaces unsupported national sizing ranges, current tariff claims and purported customer projects with project-specific checks and clearly hypothetical arithmetic. Electrical design, installation and commissioning need qualified responsibility; this is not a wiring procedure.

1. Define the Operating Objective

For a retrofit, record the existing inverter, PV strings, meter arrangement, service phase, connection agreement and usable installation space. For a new system, compare a solar-only baseline with several battery sizes under the same load, weather and tariff assumptions.

Backup is a separate design requirement. A hybrid inverter or battery does not by itself prove that a home can operate during an outage. Confirm the manufacturer’s documented isolation and backup configuration, backed-up circuits, continuous and starting power, and whether solar recharge or restart is available under those conditions. See the backup battery design guide.

2. Build a Time-Aligned Load and Generation Model

Request smart-meter interval data where available, the bill’s tariff details, and an explanation of missing periods. Record expected changes such as an EV, heat pump, home-working schedule or extension. An annual kWh total cannot identify the evening demand peak or whether a battery has enough time to charge.

Use a generation model that accounts for the actual roof orientations, shading, installed DC capacity and inverter limits. Match its time zone and intervals to the load data. Distinguish measured consumption from a synthetic profile, and observed weather from a representative weather year.

For each interval, track direct solar use, surplus PV, battery charge and discharge, grid imports, exports and curtailed energy. Do not treat every exported kWh as available to storage if charging power, capacity or control rules prevent capture. Test winter and summer conditions; an annual average does not prove winter autonomy.

Keep these quantities separate:

• PV capacity in kWp DC and inverter output in kW AC.
• Battery nominal, usable and delivered energy in kWh at stated boundaries.
• Battery charge/discharge power in kW and its permitted duration.
• State-of-charge floor, everyday backup reserve and outage-start SOC.
• One-way conversion efficiency and full-cycle round-trip efficiency.

Do not apply a depth-of-discharge factor twice to capacity already quoted as usable. Likewise, a delivered-AC energy rating should not receive the same inverter loss again. Equipment limits vary by model, temperature, SOC and operating mode.

3. Check Power and Energy Independently

A preliminary energy calculation can illustrate the distinction. Suppose selected loads require 6 kWh delivered AC energy. Assume, for this hypothetical example only, an 80% nominal-energy window and 95% one-way delivery efficiency:

Required nominal DC energy = 6 / (0.80 × 0.95) = 7.89 kWh.

That result does not select a product. If the concurrent load is 4 kW but the proposed system can supply only 3 kW in the relevant mode, adequate kWh still does not meet the power requirement. Starting loads, actual SOC, aging, auxiliary consumption and recharge need separate checks. Round-trip efficiency is not automatically the discharge factor for an already charged battery.

Compare equipment on documented usable energy, continuous/peak power and duration, compatible inverter/firmware combinations, expansion limits, environmental installation conditions, warranty exclusions and service arrangements. Chemistry alone does not establish cycle life, safe location or cold-weather performance. Obtain the exact battery and inverter manuals rather than generic LFP/NMC benchmarks.

4. Confirm the Serving Network’s Connection Route

Connection requirements need the actual equipment and premises arrangement. Do not classify a project using PV DC capacity or an export cap alone.

National Grid Electricity Distribution’s single-premises G98 guidance describes a qualifying fully type-tested microgeneration route with notification within 28 days after commissioning. It is not a universal instruction to wait 28 days before installation. Check the serving network’s current criteria, forms and treatment of existing generation and storage before proceeding.

The familiar single-phase calculation 230 V × 16 A / 1,000 = 3.68 kW assumes unity power factor. It does not establish that every installation at that number qualifies for G98, or that 3.68 kWp of PV is the relevant system boundary.

National Grid’s export-limitation guidance describes a G99 fast-track arrangement involving qualifying generation/storage and G100 export control. An export cap does not turn otherwise larger equipment into a G98 installation. Obtain any required approval before connection and follow the approved commissioning procedure.

Prepare a controlled submission pack containing:

• Existing and proposed generation/storage ratings and operating arrangement.
• Exact model and type-test documentation, protection and control details.
• Single-line diagram, phase arrangement and meter/measurement locations.
• Proposed import/export limits and the approved export-limitation design where relevant.
• Connection decision, conditions, commissioning records and notifications.

Do not promise a national G99 turnaround time. Distinguish the supplier who bills electricity from the network operator handling the connection. Great Britain examples should not automatically be applied to Northern Ireland; confirm the relevant local rules and authority.

5. Treat Certification, SEG and VAT as Separate Questions

Ask the installer for its current certification scope, applicable installation standards, product documentation and completion certificates. A grid connection route, installation certification, VAT treatment and export-payment eligibility are different checks. Do not substitute a blanket “MCS-compliant” label for the documents each party requires.

Export payments and storage

Ofgem’s SEG guidance covers qualifying generation in Great Britain, including solar PV up to the scheme’s stated capacity limit. Apply to a SEG supplier using its current application process; the export-payment supplier can differ from the import supplier. The tariff must remain above zero, but there is no universal 1p/kWh minimum or guaranteed 2026 rate.

Ofgem’s storage guidance, Appendix 3, distinguishes eligible stored generation from grid-charged or mixed-origin energy. A SEG supplier is not obliged to pay for unidentifiable mixed-origin exports under the scheme. That is different from a universal ban on a supplier offering commercial terms for such exports. Confirm the tariff’s actual storage and metering conditions; an interval export meter alone does not prove the source of every unit.

The customer handoff should record the application route, meter identifiers, required certificates, storage operating arrangement, contractual export rate and start date. Do not present an OVO application query as evidence for a generic battery-sizing rule or reproduce an unverified supplier form.

VAT treatment

HMRC VAT Notice 708/6 includes qualifying installed electrical storage batteries, including standalone grid-storage installations, from 1 February 2024. The temporary zero rate is currently scheduled through 31 March 2027, with a reduced rate from 1 April 2027 under the notice’s rules. Supply-and-install treatment differs from retail purchase of equipment alone.

Check the premises, supply and installation scope with the supplier rather than assuming all battery purchases are zero-rated. Obtain an itemised invoice and its tax basis. Do not combine historic grants or loans with current eligibility without checking the administering body’s current terms.

6. Compare Tariff Scenarios Without Double Counting

For solar-charged storage, the relevant value includes avoided import expenditure minus foregone export income, cycle losses and applicable operating costs. For grid charging, include the actual cost of charging energy. Do not add the value of both strategies for the same stored batch of energy.

Keep export payments separate from avoided imports. A standing charge that remains payable is not avoided simply because the home imports less. Include changing tariffs, degradation, reserve settings, replacements and finance costs in sensitivity cases. See the solar finance guide.

Retain the original finance inputs as a hypothetical accounting example, without claiming an Edinburgh project or current offer:

The loan funds part of the purchase; it does not reduce its £10,950 capital cost. £10,950 / £1,065 = 10.28 years is only an undiscounted price-to-benefit ratio, excluding interest, repayments, degradation, maintenance and replacements. Dividing the cash contribution alone by the benefit would misrepresent the whole-project economics. A borrower cash-flow model must also include the outstanding loan and repayments.

7. Issue a Reviewable Customer and Commissioning Pack

Show the solar-only baseline, the selected storage alternative and a sensitivity case. Disclose load/weather sources, equipment models, capacity boundaries, operating strategy, tariff date and exclusions. Explain what is backed up and what remains unavailable in an outage.

At handoff, retain the accepted network conditions, commissioning results, model settings, certificates, warranties, emergency information and monitoring access. Confirm that field settings and equipment match the released design. A software estimate is not evidence that the network or installation has approved the project.

Use the battery software evaluation guide to test whether a tool can reproduce these assumptions and outputs. When evaluating SurgePV through a product demonstration, request evidence for the exact storage workflow you need rather than assuming UK tariff dispatch or automatic connection approval.

Frequently Asked Questions

What size battery does a UK home need?

Use the household load profile, available charging energy, operating strategy and required power. Bedroom count and annual consumption alone cannot establish usable capacity or backup capability.

Is G98 a 28-day waiting period before installation?

National Grid Electricity Distribution describes notification within 28 days after commissioning for its qualifying single-premises G98 route. Confirm the serving network’s criteria and any required prior approval for the actual installation.

Does an export limit automatically make a system G98 compliant?

No. Equipment ratings, existing generation, type testing and the approved connection arrangement matter. Export-limited projects can require a G99 route and approved export-control design.

Can a battery earn SEG payments?

Eligible stored generation can receive payments subject to the supplier’s conditions. Grid-charged or mixed-origin exports need careful eligibility and contract review; a supplier is not obliged to pay for unidentifiable mixed-origin energy under SEG.

Is every UK battery purchase zero-rated for VAT?

No. HMRC distinguishes qualifying installed batteries from equipment purchased alone. Check the premises and supply scope; the temporary zero rate is currently scheduled through 31 March 2027.

Does a loan reduce the installed system cost?

No. A loan funds the purchase and creates repayment obligations. Keep whole-project cost and benefits separate from borrower cash flow, including interest and repayments.